// grab all the user input buttons
let buttons = document.getElementsByClassName('user-button');
// main output field
let output = document.getElementById('current-output');
// keeps track of whether the current number has a decimal, and won't allow two decimals
let dec = false;
// holds the result of all arithmetic operations up to this point
let currentTotal = undefined;
// holds the most recent arithmetic operations
let currentArith = undefined;
// determines whether to replace the output string or edit it; either 'replace' or 'edit'
let calcState = 'replace';

window.addEventListener('click', function(e){
  if (buttons[e.target.id]) {
    // then we have the button!
    switch (e.target.getAttribute('data-btn-type')) {
      // handle all potential number entries, including decimals
      case 'number':
        numberEntry(e.target);
        break;
      // handle all potential edits: all clear, clear entry, and backspace
      case 'edit':
        editEntry(e.target);
        break;
      // handle calculations
      case 'calc':
        calculateEntry(e.target);
        break;
    }
  }
});

// number entry accepts a target button element (e.target) and alters the main
// output field based on the target's contents and the calc state
function numberEntry(t) {
  let text = calcState == 'edit' ? output.textContent : '';
  // handle decimals
  if (t.id === 'btn-dec') {
    if (dec) {
      return;
    }
    else {
      if (text === '' || calcState === 'replace') {
        // if the current output string is blank, set it to '0'
        text = '0';
      }
      // keep track of the decimal's position in the output number string
      dec = text.length;
      text += t.textContent;
      output.textContent = text;
      calcState = 'edit';
      return;
    }
  }
  // handle number inputs; if the output text field is showing 0, replace that value
  if (text === '0' && !dec) {
    text = '';
  }
  text += t.textContent;
  output.textContent = text;
  calcState = 'edit';
}

// alter the current main output with backspace, clearing the current entry, or
// clearing all calculations. accepts a target button element (e.target)
function editEntry(t) {
  let text = calcState == 'edit' ? output.textContent : '0';
  // handle backspaces
  if (t.id === 'btn-back') {
    // if backspacing the last number, set the output field to '0'
    if (text.length === 1) {
      text = '0';
      output.textContent = text;
      calcState = 'replace';
      return;
    }
    // otherwise, remove the last character
    else {
      text = text.slice(0, text.length - 1);
      // if the output string's length is now lower than the decimal's last position,
      // update the decimal value to false
      if (text.length - 1 < dec) {
        dec = false;
      }
      output.textContent = text;
      return;
    }
  }
  // clear entry and all clear both zero out the output field and reset the decimal value
  if (t.id === 'btn-ce' || t.id === 'btn-ac') {
    text = '0';
    dec = false;
    calcState = 'replace';
  }
  // all clear also erases the arithmetic and number history
  if (t.id === 'btn-ac') {
    currentTotal = undefined;
    currentArith = undefined;
  }
  output.textContent = text;
  return;
}

// add the current output field plus the chosen operation to the equation
// also sums the equation, and updates the output field. accepts a button target (e.target)
function calculateEntry(t) {
  // handle equals
  if (t.id == 'btn-eq') {
    if (currentTotal === undefined && calcState === 'replace') {
      console.log('nope!');
      return;
    }
    console.log('getting total from equals');
    output.textContent = getTotal(currentTotal, currentArith, output.textContent);
    currentTotal = undefined;
    calcState = 'replace';
    dec = false;
    return;
  }
  // handle user selecting operation before any values are entered (initial/all clear states)
  if (currentTotal === undefined && calcState === 'replace' && output.textContent === '0') {
    console.log('cannot perform ' + t.textContent + ' operation: initial calculator state');
    return;
  }
  
  // handle valid calcState and valid user input, but no currentTotal
  if (currentTotal === undefined) {
    currentTotal = output.textContent;
    currentArith = t.textContent;
    dec = false;
    calcState = 'replace';
    console.log('line 132: ', currentTotal, currentArith);
    return;
  }
  
  // handle user entering multiple operations after one another
  if (calcState === 'replace' && (
    t.id === 'btn-div' ||
    t.id === 'btn-mult' ||
    t.id === 'btn-sub' ||
    t.id === 'btn-add' 
  )) {
    currentArith = t.textContent;
    dec = false;
    calcState = 'replace';
    console.log('line 152: ', currentTotal, currentArith);
    return;
  }
  
  if (currentTotal !== undefined && calcState === 'edit' && currentArith !== undefined) {
    currentTotal = getTotal(currentTotal, currentArith, output.textContent);
    output.textContent = currentTotal;
    currentArith = t.textContent;
    dec = 'false';
    calcState = 'replace';
    console.log('line 161: ', currentTotal, currentArith);
    return;
  }
}

function getTotal(a, operator, b){
  let total;
  if (a === undefined || operator === undefined || b === undefined) {
    total = a || b;
  }
  else {
    total = eval(a + operator + b);
  }
  return total;
}